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Volt \Μονάδα
Volt The volt (symbol: V) is the SI derived unit of electric potential difference or electromotive force Ετυμολογία It is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, the first chemical battery. Ορισμός The volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. Hence, it is the base SI representation m2 · kg · s-3 · A-1, which can be equally represented as one joule of energy per coulomb of charge, J/C. : \mbox{V} = \mbox{W} \cdot \mbox{A}^{-1} = \mbox{J} \cdot \mbox{C}^{-1} = \mbox{m}^2 \cdot \mbox{kg} \cdot \mbox{s}^{-3} \cdot \mbox{A}^{-1} . Josephson junction Ορισμός Since 1990 the volt is maintained internationally for practical measurement using the Josephson effect, where a conventional value is used for the Josephson constant, fixed by the 18th General Conference on Weights and Measures as :K{J-90} = 0.4835979 GHz/µV. Υδραυλικό Ανάλογο In the hydraulic analogy sometimes used to explain electric circuits by comparing them to water-filled pipes, voltage is likened to water pressure - it determines how fast the electrons will travel through the circuit. Current (in amperes), in the same analogy, is a measure of the volume of water that flows past a given point, the rate of which is determined by the voltage, and the total output measured in watts. The equation that brings all three components together is: volts × amperes = watts Συνήθη voltages Nominal voltages of familiar sources: * Nerve cell action potential: around 30 millivolts * Single-cell, rechargeable alkaline battery: 1.25 volt * Mercury battery 1.355 volts * Single-cell, non-rechargeable battery (e.g. AAA, AA, C and D cells): 1.5 volts * Lithium polymer rechargeable battery: 3.75 volts * Transistor-transistor logic (TTL) power supply: 5 volts * PP3 battery: 9 volts * Automobile electrical system: 12 volts (nominal) * Household mains electricity: 230 volts RMS in Europe, Australia, Asia and Africa, 120 volts RMS in North America, 100 V RMS in Japan (see List of countries with mains power plugs, voltages and frequencies) * Rapid transit third rail: 600 to 700 volts (see List of current systems for electric rail traction) * High voltage electric power transmission lines: 110 kilovolts RMS and up (1150 kV is the record as of 2005) * Lightning: Varies greatly, often around 100 megavolts. Note: Where 'RMS' (root mean square) is stated above, the peak voltage is \sqrt{2} times greater than the RMS voltage for a harmonic signal (sine or cosine wave). Ιστορική Αναδρομή In 1800, as the result of a professional disagreement over the galvanic response advocated by Luigi Galvani, Alessandro Volta developed the so-called Voltaic pile, a forerunner of the battery, which produced a steady electric current. Volta had determined that the most effective pair of dissimilar metals to produce electricity was zinc and silver. In the 1880s, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt for electromotive force. The volt was defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. Prior to the development of the Josephson junction voltage standard, the volt was maintained in national laboratories using specially constructed batteries called standard cells. The United States used a design called the Weston cell from 1905 to 1972. Εσωτερική Αρθρογραφία * SI for unit prefixes * SI electromagnetism units Category: Μονάδες Μέτρησης SI